Organic light emitting devices (OLEDs) are emissive displays consisting of a transparent substrate coated with a transparent conducting material, such as Indium Tin oxide (ITO), one or more organic layers, and a cathode made by evaporating or sputtering a metal of low work function characteristics, such as Ca or Mg. The organic layers are chosen so as to provide charge injection and transport from both electrodes into the electroluminescent organic layer (EL) where the charges recombine, emitting light. There may be one or more organic hole transport layers (HTL) between the ITO and the EL, as well as one or more electron injection and transporting layers (EL) between the cathode and the EL.
OLEDs hold out the promise of providing inexpensive displays. In principle, these devices can be manufactured on flexible substrates and fabricated using "roll-to-roll" processing equipment. Inexpensive equipment for such fabrication operations such as polymer film coating devices, metal evaporators and lithography equipment capable of providing the deposition of the various layers are already available. For example, Web coating devices for thin polymer films that are a few feet wide can operate at a feed rate of hundreds of feed per minute.
To function over extended periods of time, an OLED must be sealed to prevent water and oxygen from reaching the cathode and polymer layers. Unfortunately, polymers having sufficiently low permeability to water and oxygen are not available. For example, Poly(ethylene terephthalate) or PET, which is used as a command substrate for Web processing has a water permeability that is so high that devices constructed thereon begin to degrade almost immediately due to reaction of water from the air with the cathode material. Accordingly, some form of sealant coating must be applied to the polymer to achieve the required resistance to water and oxygen. In addition, the cathode layer must be sealed on the other side of the device to prevent water and oxygen from entering from that side and destroying the cathode.
One coating technique that has shown promise is the Polymer Multilayer (PML) technique described in U.S. Pat. Nos. 4,842,893, 4,954,371, and 5,260,095. In this technique, a coating consisting of a layer of polymer and an layer of an aluminum oxide is applied to the flexible substrate to seal the substrate. Both the deposition steps can be operated on Web processing equipment at very high speeds. While the resistance to water and oxygen permeation is improved by three to four orders of magnitude relative to uncoated PET films, the resulting films are still sufficiently permeable to limit the lifetime of the OLEDs in application requiring lifetimes of several years and/or exposure to hot humid environments. Using accelerated lifetime test procedures, it can be shown that the permeation rate should not exceed about 4.times.10.sup.-7 moles H.sub.2 O/m.sup.2 day in order to have a storage lifetime of 10 years. The best films currently available have permeabilities that are at least a factor of 50 too high. It should be noted that applying several polymer bilayers does not improve the resistance to water and oxygen sufficiently to provide the required increase in resistance.
Broadly, it is the object of the present invention to provide an improved OLED display and method for making the same.
It is a further object of the present invention to provide a method for constructing a PML that has sufficient resistance to water and oxygen permeation to provide OLEDs having commercially useful lifetimes.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.